Couldn't it be well expected that, encrypted data aside, files with same content are often used by more than one persona?
> Greatly reduce backup & sync time through comprehensive compression and advanced de-duplication (saving you time)
> You are only charged for the compressed de-duplicated data amount (saving you money)
Still it is not clear if they do cross-user deduplication, but I think it is very unlikely because all the content is encrypted with an user-specific key, which I think they don't have access to.
I seem to recall that Dropbox (or another well known online storage startup) implements this strategy. Maybe it works.
Videos, audio and photos and game media will take the bulk of the space. Of those - only photos and a proportion of the videos are likely to be totally unique to a user.
Consider that Dropbox gives you 50gb for the basic plan. I'm guessing most people don't back up videos, games or their OS using that space, but rather back up their documents, projects they're working on in whatever field, photos, and music.
Of those, only with music is there a chance to use deduplication, and that's assuming you can figure out two music files with different ID-tags are the same.
(Come to think of it, in my Dropbox, music easily takes up 70% of my quota, so maybe is is worthwhile after all.)
It would be interesting for Dropbox to release numbers on how many music files are identical between different people.